The enzymes responsible for repairing DNA damage also function in several biologically significant pathways such as DNA replication, genetic recombination, and mutation fixation; and are thus directly implicated in the oncogenic process. Not only is little known about the repair of ionizing radiation induced dmages in DNA, but the chemical nature of these damages remains to be elucidated. The proposed research would be undertaken in an effort to define the molecular mechanisms involved in the repair of X-ray induced damages in DNA. More specifically, the in vitro repair of X-ray damaged T4 DNA will be examined using purified enzymes known to be involved in repair pathways. In order to compare in vitro to in vivo repair processes enzymatically repaired T4 DNA will be assayed on a variey of Escherichia coli hosts with defined repair capabilities. Also, the properties of an X-ray specific endonuclease from E. coli will be defined, and T4 infected cells will be screened for similar activity. The biosynthetic capacities of T4x and T4y which are sensitive to ionizing radiation and deficient in genetic recombination, will also be studied with an end to isolating the x and y gene products. Lastly, an effort to isolate a presumptive T4 X- ray sesitive mutant will be continued.